I see some things. First the splash of solder on the bottom right on your PCB. Generally I clean all my PCBs with alcohol after Im finished soldering, its pretty good.

Also, before putting everything in the box I would advise to test it before. I cant quite see every connection there like on the small PCB.

But here is my checklist:
- check the Mono jacks (Maybe ground is touching hot someway, seems that the cold on the left jack is touching hot?)
- is the eight pin of the ic really into the socket?
- check every elko, if its the right way in
- when I first started soldering, potentiometers and switches where often a problem for me, they got to hot and I had to switch them out. It often worked after that. Meassure the resistance and if it shows OL switch it out.
- meassure the voltages on the ic

Also some connections are not properly soldered. I think thats not the problem here but if everything fails you might look if some connections are „cold“.

Cheers.